<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>

<body>
    <div id="app">
        <input type="text" name="" id="" v-model='num1'>
        <select name="" id="" v-model='operato'>
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
            <option value="000">/</option>
        </select>
        <input type="text" v-model='num2'>
        <input type="button" value="=" @click='calc'>
        <span>{{sum}}</span>
    </div>
</body>
<script>
    const vm = new Vue({
        el: '#app',
        data: {
            num1: '',
            num2: '',
            sum: '',
            operato: '+'
        },
        methods: {
            calc() {
                this.num1 = Number(this.num1)
                this.num1 = Number(this.num2)
                switch (this.operato) {
                    case '+':
                        this.sum = this.num1 + this.num2
                        break;
                    case '-':
                        this.sum = this.num1 - this.num2
                        break;
                    case '*':
                        this.sum = this.num1 * this.num2
                        break;
                    case '/':
                        this.sum = this.num1 / this.num2
                        break;
                    default:
                        break;
                }
            }
        }
    })
</script>

</html>